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ON THE  CHANGE  OF EEFRANGIBILITY OF LIGHT.          263
do show may not be due to some impurity. Among fluids which exhibit the phenomenon in a high degree, or according to the author's expression are highly sensitive, may be mentioned a weak decoction of the bark of the horse-chestnut, an alcoholic extract from the seeds of the Datura stramonium, weak tincture of turmeric, and a decoction of madder in a solution of alum. In these cases the general character of the dispersion resembles that exhibited by a solution of sulphate of quinine, but the tint of the dispersed light, and the part of the spectrum at which the dispersion begins, are different in different cases. In the last fluid, for example, the dispersion commences somewhere about the fixed line D, and continues from thence onwards far beyond the extreme violet. The dispersed light is yellow, or yellowish orange.
In the case of other fluids, however, some of them sensitive in a very high degree, the mode in which light is dispersed internally presents some very remarkable peculiarities. One of the most singular examples occurs in the case of an alcoholic solution of the                     *j
green colouring matter of leaves.    This fluid disperses a rich red                     ||
light.    The dispersion commences abruptly about the fixed line J5,                     |l
and continues from thence onwards throughout the visible spectrum                     f ]
and a little beyond.     The dispersion is subject to fluctuations                         ,'jj
intimately connected with the singular absorption bands exhibited                     *
by this medium.
In order that a medium should be capable of changing the refrangibility of light incident upon it, it is not necessary that the medium should be a fluid, or a clear solid. Washed papers and other opaque substances produce the same effect, but of course the mode of observation must be changed. The author has observed the change of refrangibility in various ways. It will be sufficient to mention here that which was found most generally useful, which he calls the method of observing by a linear spectrum. The method is as follows.
A series of prisms and a lens are arranged in the usual manner for forming a pure spectrum, but the slit by which the light enters, instead of being parallel, is placed in a direction perpendicular to the edges of the prisms. A linear spectrum is thus formed at the focus of the lens, consisting of an infinite succession of images of the slit arranged one after the other in the order of refrangibility, and of course overlapping each other to a certain extent. The substance to be examined is placed in the linear spectrum, and